The effect of the environmental pollutant and toxicant methylmercury on chromosome structure and function will be studied in cells of the continuous human cell line HeLa S3 (1) by analyzing, subsequent to methylmercury treatment, the fine structure as well as transcriptional activity of the cells' dispersed form of interphase nuclei (chromatin) with the aid of 'mapping' techniques (two-dimensional gel electrophoresis); (2) by monitoring the activity of the heavy metal-inducible metallothionein genes based on their capability to produce metallothionein mRNAs as well as metallothionein; and (3) by studying, in an extracellular system, the physico-chemical properties of nucleosomes, reassembled from the constituent chromosomal histone and non-histone proteins and DNA in the presence and absence of methylmercury, with the aid of thermal denaturation and circular dichroism measurements. It is the ultimate aim of this plan to provide, at the molecular level, a rationale for the finding that organomercurials in general, and methylmercury in particular, are powerful in vivo clastogens and teratogens (in addition to their also being powerful neurotoxicants).